Some residents of Perth, Australia, receiving treated groundwater have complained of intermittent swampy, cooked vegetable odours in their drinking water. The compound primarily responsible is dimethyl trisulfide (CH3SSSCH3) which can be formed in the laboratory from the reaction of methyl iodide with thiosulfate (in the presence of iodine or sulfide) or with polysulfide (Sn2−). Addition of chlorine or sulfide to pre-formed S-methylthiosulfate (produced from the reaction of methyl iodide with sodium thiosulfate) at concentrations possibly expected in water supplies did not produce the swampy odour or only formed it slowly. Dimethyl trisulfide was formed upon the addition of methyl iodide to samples from each water supply system examined, even from those not prone to the formation of swampy odour, suggesting each contained polysulfide. Polysulfides and the reducing agents sodium oxalate, sodium borohydride and sodium thiosulfate reacted with dimethyl trisulfide in solution, removing 50-100% of that originally present.